问题
In the C programing language, why do the bitwise operators (& and |) have lower precedence than the equality operator (==)? It does not make sense to me.
回答1:
You need to ask Brian Kernighan or Dennis Ritchie.
From this forum: http://bytes.com/topic/c/answers/167377-operator-precedence
The && and || operators were added later for their "short-circuiting" behavior. Dennis Ritchie admits in retrospect that the precedence of the bitwise operators should have been changed when the logical operators were added. But with several hundred kilobytes of C source code in existence at that point and an installed base of three computers, Dennis thought it would be too big of a change in the C language...
So, that might be a reason? I'm guessing since there are several layers of bitwise precendence (unlike relational comparisons) that it's cruft that's existed since...forever...and just was never corrected.
回答2:
It doesn't make sense to Dennis Ritchie, either, in retrospect.
http://www.lysator.liu.se/c/dmr-on-or.html
&& and || were added to the language after | and &, and precedence was maintained for reasons of compatibility.
回答3:
I don't have an authoritative answer as to why K&R chose the precedence they did. One example that makes a fair amount of sense would be this one:
if (x == 1 & y == 0) {
/* ... */
}
Since this is the bitwise AND operator it uses a non-short-circuiting evaluation mode, as would
if (x == 1 | y == 0) {
/* ... */
}
use the non-short-circuiting OR operator. This is probably why they chose to have the precedence group this way, but I agree with you that in retrospect it doesn't seem like a good idea.
来源：https://stackoverflow.com/questions/4685072/operator-precedence-bitwise-lower-than